Project Summary Normal postnatal lung morphogenesis and repair after injury requires the precise coordination of cellular activities and matrix remodeling for the formation or restoration of normal alveolar structures and optimal respiratory efficiency. We recently identified a critical role for the protease, Chymotrypsin-like elastase 1 (Cela1), in the regulation of postnatal lung elastance and its critical role in the pathogenesis of ?1-antitrypsin (AAT) deficiency. This program of research is based on our novel preliminary findings that Cela1 expression is induced during regenerative and pathological alveolar remodeling, and that inhibition of Cela1 activity prevents AAT deficiency related emphysema. This proposal will identify the role of Cela1 in both physiological and pathologic remodeling in the postnatal lung. Based on data that Cela1 expression is dysregulated in hyperoxic lung injury, Aim 1 will define cell-specific roles for Cela1 in lung development using conditional deletion models to identify Cela1-expressing cells and characterize dynamic changes in expression during development under normal and hyperoxic conditions. Aim 2 will test whether cell-specific expression of Cela1 changes in pathological lung matrix remodeling and whether targeting Cela1 can protect against emphysema long-term. Using lineage tracing and proximity ligation in situ hybridization in an AAT-deficient emphysema model, we will determine whether Cela1-expressing ATII cells represent a unique epithelial cell subclass or if all ATII cells can express Cela1 in the appropriate context. In Aim 3, whether AAT neutralization of Cela1 is required for cellular uptake will be tested, and the cells involved in this uptake and recycling of AAT-Cela1 complexes will be identified using lineage tracing, proximity ligation in situ hybridization, confocal microscopy, and flow cytometry. The association among Cela1 gene expression and regions of elastin remodeling will be identified in human AAT deficient emphysema specimens. Lastly, site of Cela1-AAT molecular interaction will be defined. From a scientific and clinical standpoint, this proposal will define a novel critical mechanisms by which Cela1 mediates matrix remodeling processes to regulate normal alveolarization and regeneration after injury that will provide a strong rationale to explore Cela1 as a target for development of future therapies for interstitial lung diseases.